1. Field of the Invention
This invention relates generally to image processing for reconstruction of images on a display device. The invention more particularly relates to signal processing of color image data such as color video data for compression of image data to be transferred from an encoding site to a decoding site in reconstruction of color images.
2. Description of Related Art:
Ordinary telephone lines can generally carry a digital video signal at a maximum rate of about 18,000 bits per second. Digital color video coding and transmission techniques have been developed which can transmit full color still images over ordinary telephone lines, with visually acceptable quality of individual images, at data rates as low as about 11,500 bits per second. However, the amount of complexity or detail in the images to be encoded and transmitted, and the real time rate and degree of changes from one image to the next, can severely affect the capability of such techniques for encoding and transmitting color images at a visually acceptable rate without appearing as jerky motion.
Image information data compression techniques applicable to black and white video images as well as color video images have been developed which involve the reduction of redundant and non-essential information in transmitting sequences of images. One such technique involves transmitting only those parts of an image which are selected to be important or which are determined to have changed in a significant manner from a previously transmitted image. A drawback of such methods is that they can also significantly reduce the resolution of an image.
One technique useful in reduction of the time necessary to transmit a given image is known as run length encoding. This technique is described in U.S. Pat. Nos. 3,609,244 (Mounts) and 4,420,771 (Pirsch). In run length encoding, scan lines of an image are encoded as a length of a sequence of pixels having common values or ranges of values for one or more given components of the pixels, such as luminance or chrominance. One method for compressing color video data for generation of a sequence of color video images involves forming a map of color codes according to a subjective selection of the most visually significant combinations of color components in a color image. This method is described in U.S. Pat. Nos. 4,857,993 and 4,914,508 to Music et al., which are incorporated herein by reference. A technique for improving the quality of images generated with run length color information smooths color transitions by interpolating between endpoints of run lengths, as is described in U.S. Pat. No. 4,843,466 (Music et al.), which is also incorporated herein by reference.
It would be desirable to provide a method for generating a color palette directly from data from one image, for matching colors in other images in a sequence, to allow high quality color matching, and to reduce the amount of information needed for real time reconstruction of sequences of color images. It would also be desirable to provide a technique for determining a selected number of the most frequently occurring colors in an image, to allow for selection of different modes of color image reconstruction balancing the number of most frequently occurring colors in an image with the degree of compression and speed of transmission of image sequences. It would also be desirable to provide a faster, simpler method of pixelization of run length information for smoothing color transitions between the color values at the end points of run lengths, to enhance both the quality and speed of color image reconstruction.